I haven't had the chance to do a back to back comparison but it MAY be the target market. My LS400 VVTi throttle is my least favorite part of the driving experience. I think that Lexus did not want to scare some of its owners with too quick of a responding car.
There is a 10hp/10tq difference in ratings between the GS400 and LS400. I am pretty sure that the drivetrain (engine, trans, differentials) are mechanically almost identical with only the air cleaner to throttle body and the exhaust being the difference. ECU programming may well be tuned slightly sportier in the GS than the LS and probably the SC as well.
The LS has very strange angle cut and welded exhaust tubing at the catalyst outlet. It starts as 60mm for apx 2", is angle cut and welded apx 45 degrees, from the welded angle it remains 60mm for apx 2" or so before swaging down to 50mm. I do not like this type of transition. Very odd engineering. From there it is identical to the GS400 Y pipe until immediately after the merge where the LS is at first 60mm then almost immediately necks down to 55mm and goes through the 55mm center resonator, it then expands back to 60mm for a foot or so before splitting back into dual 50mm pipes going into the strange side entry back exit center mufflers.
The GS has 50mm right off the catalyst outlets which IMO is better than the turbulent angle cut LS design. These merge into a 60mm center going into a 60mm center resonator and then into a formed splitter followed by dual 55mm straight through resonators. The SC has the WORST of both worlds with single tube exhaust manifolds and restrictive exhaust vs GS.
The final part MUST include the way the car is predominently driven. The ECU really does modify the engines performance. When I first went to the racetrack the best I could manage was a 14.9 ET at 95mph. After a week of aggresive driving and practicing various launching techniques I was able to run 14.6 at 97 consistently. The next week with the rear mufflers off and air cleaner mods I was into 14.4 and 14.5. A couple later with perfect conditions it ran my best of 14.22 at 99.38mph which is a HUGE difference from 14.9 at 95. Typically it takes 50+ HP difference for this performance change and it was NOT due to the mufflers (that only saves weight).
It's good to mention the LS has a different exhaust angle from the SC. John, you're the exhaust man. Lol.
From the LS exhaust design with less restriction, I think it flows better than the SC. Also, the gear ratio of the LS may be the reason it's faster at low end than the SC. Supposedly, Toyota should put all the best stuffs into the SC, because it's their only Lexus coupe. I guess not. The only good thing in the SC is it has better handling than the LS, and a sporty feel inside the seats. The SC even has an inferior braking system comparing to the LS.